The role of population genetics in forensic DNA analysis is in building the foundation of the wide range of databases obtained from a population and telling us the likelihood of the source of the sample obtained from a crime scene.

Regarding the comparison of DNA from a crime-scene specimen and from a suspect, under current procedures, if the DNA profile from the crime-scene sample reportedly matches that of the suspect, there are two possibilities (aside from error): The DNA at the crime scene came from the suspect or the DNA at the crime scene came from someone else who had the same profile as the suspect.

If the DNA profile in question is common in the population, the crime-scene DNA might well have come from someone other than the suspect.

If it is rare, the matching of the two DNA profiles is improbable to be a mere coincidence; the rarer the profile, the less likely it is that the two DNA samples came from different persons.

Figure 1 - Geneology in Forensics (Image Source- sciencenews.org)

To assess the probability that DNA from a randomly selected person has the same profile as the evidence DNA, we must know the frequency of that profile in the population.

A very small proportion of the trillions of possible profiles are found in any database, so it is necessary to use the frequencies of individual alleles to estimate the frequency of a given profile.

That approach necessitates some assumptions about the mating structure of the population.

Populations and Their Ancestral Roots 

Studying population genetics using autosomal STR markers is quite popular despite the relevant developments in SNP analysis because of their power of discrimination used in decrypting mixture components and the ability to be combined in multiplex assays in order to recover information from low quantities of biological samples.

Since they are highly variable in nature, they are considered to be a powerful method to distinguish individuals at a forensic level.

Population parameters such as Power of Discrimination (PD), Polymorphic Information Content (PIC), And Power of Exclusion (PE) are calculated for analyzing the microlevel genetic structure of any population using 15 or more autosomal STR loci. Software like Arlequin is used to obtain the heterozygosity, locus-by-locus AMOVA, genetic diversity, and pairwise Fst values. 

Bioinformatics software and tools are routinely employed to interpret such data from various populations to study their diversity in their genetic make-up.

The study of autosomal STRs also evaluates the flow of genetic information, isolated traits, and reduced variation, which is a result of genetic drift, and/or founder effect on small remote populations, especially tribal populations, where endogamy is predominantly practiced. 

The Alu family of SINEs is spread throughout the primate lineage and is the predominant SINE within the human genome.

Unlike other DNA markers, their distribution and possibly other SINE loci and LINEs allow tracing of population ancestry.

Determining lineage and ancestry also includes the Y-STR and mitochondrial (mt-) DNA typing. In paternity testing, if the child possesses a DNA strand that isn’t present in either the mother or father (suspected), then the man is not the child’s biological father and paternity for the suspect is excluded.

Mitochondrial DNA proves to be a very useful tool in identifying perpetrators and human remains and for determining ancestry.

The coding region mutation in mtDNA is believed to occur approximately once every 6500 years.

Thus, mtDNA is extremely stable, which implies that we can accurately trace our maternal ancestors from a thousand years ago across generations. Y-STR DNA has a similar principle except that it is located in the male Y chromosome and is passed down along the paternal line of ancestry. Since mtDNA is tough, scientists usually extract it from teeth and bones of very old skeletal remains and use it for the identification and determination of their ancestry.

mtDNA can be found in places where nuclear DNA is not found, for instance, hair.

If hair from a crime scene is found to match the suspect’s hair physically the IOs may hypothesize the suspect’s presence in the scene. The problem arises when these “proofs” are not foolproof and are wobbly at best.

But if the mtDNA is matched the evidence gains more power to suggest that the hair must have come from the suspect or at least someone sharing the same maternal ancestry. 

Familial DNA 

Familial DNA analysis is done when evidence in an investigation doesn’t provide a much stronger lead or is not enough to a strong prosecution.

Often a suspect’s DNA is not available and IOs can’t get a warrant to demand a sample, they often turn to familial DNA by testing close or immediate relatives.

If the profile is found to be close to that obtained from the crime scene it may be enough to get the required warrant to coerce the suspect to provide a sample.

DNA databases can play an important role in helping to identify missing individuals over time. When a family member goes missing, DNA samples can be obtained from direct reference samples or biological relatives.

DNA profiles from these samples would then be uploaded to the database and searched against DNA profiles from unidentified human remains in an effort to make an association with a missing individual.

This area has opened up a study termed genetic genealogy.

Genetic Genealogy and Forensics 

Termed by Colleen M. Fitzpatrick, forensic genealogy, also called molecular genealogy, widens the possibilities of such identifications enabling people to find their ancestral roots and complete their family history.

It’d have been impossible without DNA fingerprinting as most of the tests used in genealogy are created for the identification of Y-STR and mt-DNA markers.

Since genes on autosomes undergo segregation and recombination the length of identity-by-descent (IBD) fragments shared between individuals of different levels of kinship are different.

The concept of “genetic triangulation” is used to describe the method of using two/more known data points to determine the Y-STR and mt-DNA ancestor haplotype: first introduced in 2004 by Bill Hurst.

Presently, this technology is used in combination with the shared IBD fragment information of autosomal STRs to construct a pedigree tree. Each “genetic triangle” consists of 3 points – sample DNA, a match, and a third individual sharing IBD fragments with the other two samples. 

Challenges To Conquer: Ethical Challenges in Forensic Genealogy 

One of the major challenges for maintaining a DNA database is the issue of privacy and security of the information stored since the database contains DNA profiles that correspond with a databank of biological samples.

DNA samples contain genetic information that could be used against an individual or his or her family if not handled properly. The issue of privacy can be approached in multiple ways.

First, the DNA markers, such as the 13 CODIS core STR loci, are in non-coding regions of the human genome and are not known to have any association with a genetic disease or any other genetic predisposition. Thus, the information in the database is only useful for human identity testing.

Second, no names of individuals or other characterizing data are stored with the DNA profiles at the national level. Specific case data are secured and controlled by the law enforcement agencies that submit the data.

Thus, only the crime laboratory that submitted the DNA profile has the capability to link the DNA results with a known individual.

Third, data are encrypted and shared through a secure network only accessible to state and local CODIS administrators. Fourth, federal and state penalties for improper use of criminal DNA samples include fines and possible imprisonment.

Advancements in Genetic Genealogy

Advances have also been made in the chemical analysis of traces of skin cells left behind on objects.

Examination of a very small amount of these epithelial cells can give the examiner information far beyond identification, including information about one’s personal life.

This goes beyond genetic information to perhaps aid in creating a composite sketch of possible physical features of a suspect.

While this would seem the natural progression of DNA and skin trace analysis, it is important to understand the distinction between the abilities of touch DNA and chemical analysis of skin traces and traditional DNA.

They are two-fold and raise significant issues as to whether the legal framework surrounding traditional DNA analysis is sufficient for this new form.

First, touch DNA and related technologies require a sample so small that it is often undetectable to the source person and is available from any item touched by a person.

Second, the amount of information that can be learned from these samples is much more intrusive into the private life of the individual.

Suspects—and indeed any source of this evidence—risk the government examining on the genetic and molecular level such samples to learn deeply private information without justification.

If the government seeks to genetically and chemically examine the vast information, it must obtain court approval. 

A number of commercial DNA databases have appeared in the US like 23andMe, for risk assessment of potential diseases, Ancestry.com and GEDmatch are websites that help find the source of family genealogy.

This data is perniciously tampered with by hackers or accessed illegally, the consequences will be unimaginable. Since different companies upload data on the basis of different protocols followed in the creation of the data.

Thus, the right to access DNA databases should be strictly controlled, penalizing illegal access to the database or tampering with the information contained therein.

Addressing the privacy risks, it’s recommended while constructing a database, intron regions related to phenotypes like eye color, skin color, height, and other human phenotypic traits are not to be selected to check leakage of personal phenotype information.

This can be well appreciated in the case of the US FBI, of which the DNA database management species that there are only index numbers and data information and pertinent personal identity details are stored in each state of the US.

If anybody wished to obtain both types of information at the same time, it is required to get the permission of both departments, greatly improving data security, reducing chances of tampering. In places of multi-ethnic and multi-genetic backgrounds, the applications of forensic genealogy should take into consideration the influence of genetic backgrounds of different ethnicities and approximately adjust and optimize the inference algorithm to ensure accuracy of the match to avoid mismatches by forensic genealogists misjudging the inferences parameters. 

Considering the complex application of forensic genealogy to infer distant relatives, it’s usually recommended to use it as a supplementary method to other identification methods, narrowing down the scope of investigation and providing clues to the police.

Compared to the use of Y-STRs the use of forensic genealogy methods to infer suspects’ distant relatives is more complex and will thus affect the inferential accuracy.

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Written by: Fulmita Deb